In order to reduce the release of exhaust emissions to atmosphere, vehicles may be equipped with various exhaust aftertreatment devices. For example, three-way catalysts may reduce levels of various emissions including carbon monoxide and unburnt hydrocarbons while selective catalyst reduction systems may be used to reduce levels of NOx. To ensure the aftertreatment devices are functioning optimally, various sensors may be installed upstream and/or downstream of the devices, and feedback from the sensors may be used to determine if emissions are slipping past the devices.
In order to determine if NOx is slipping past a catalyst, a NOx sensor placed downstream of the catalyst may be monitored, and if the sensor detects NOx in the exhaust, operating parameters may be adjusted to reduce NOx emissions in the exhaust and/or a vehicle operator may be notified that the catalyst is degraded. However, particularly when used in gasoline engines, NOx sensors may not be sensitive enough to detect the lower levels of NOx produced by the engine. Additionally, NOx sensors may be expensive, thus limiting the extent of their usage.
The inventors have recognized the issues with the above approach and offer a method to at least partly address them. In one embodiment, a method for an engine comprises operating the engine with an upstream exhaust sensor, intermediate exhaust sensor, and downstream exhaust sensor each indicating rich, adjusting engine operation to operate the engine with an upstream exhaust sensor, intermediate exhaust sensor, and downstream exhaust sensor each indicating lean, adjusting engine operation to operate the engine with the upstream exhaust sensor indicating rich and the intermediate and downstream exhaust sensors each indicating lean, and indicating degradation of an SCR catalyst based on when the intermediate and downstream exhaust sensors to switch from lean to rich.
In this way, an exhaust oxygen sensor such as a HEGO may be used to diagnose an SCR catalyst. The engine may be operated to store oxygen in the three-way catalyst, and then during a thermal event ammonia is released from the SCR catalyst. Upon a return to slightly rich operation, the stored oxygen in the TWC may react with the reductants in the exhaust from the engine to prevent the intermediate sensor, located upstream of the SCR catalyst, from sensing any reductants. The downstream sensor, located downstream of the SCR catalyst, thereby senses only the NH3 released from the SCR catalyst. Based on a timing of when the downstream sensor switches from lean to rich (which indicates the amount of ammonia released from the SCR catalyst), degradation of the SCR catalyst may be indicated. Alternatively, if the difference in time for the intermediate and downstream sensors to switch from lean to rich falls below a threshold, degradation of the SCR catalyst may be indicated.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.